Data networks provide for communication between a number of users by means of networking, i.e., connecting the individual users to one another. Communication in this context means the transmission of data between the users. The data to be transmitted are sent out as data messages, i.e. the data are packed together to form a number of packets (a/k/a “data pockets”) and sent in this form to the corresponding receiver via the data network.
The term transmission of data is here used synonymously with the abovementioned transmission of data messages or data packets. The networking itself is achieved, for example in switchable high-performance data networks (e.g., the Ethernet), in that between two users at least one switching unit is connected to both users. Each switching unit however can be connected to more than two users. Each user is connected to at least one switching unit but not directly to another user. Users are, for example, computers, stored-program controls (SPC) or other machines which exchange and/or process electronic data with other machines. In contrast to bus systems in which each user can directly reach any other user of the data network via the data bus, the switchable data networks are exclusively point-to-point connections, i.e. a user can only reach all other users of the switchable data network indirectly by forwarding of the data to be transmitted by means of one or more switching units.
In distributed automation systems, for example in the field of drive technology, particular data must arrive at the intended users thereof at particular times and be processed by the receivers. These are called real-time-critical data or data traffic since an untimely arrival of the data at the destination leads to unwanted results at the user. According to IEC 61491, EN61491 SERCOS interface—Brief technical description (http://www.sercos.de/deutsch/index_deutsch.htm), a successful real-time-critical data traffic of the said type can be guaranteed in distributed automation systems.
Various standardized communication systems, called bus systems, for the exchange of data between two or more electronic assemblies or devices are known in the prior art, and particularly for use in automation systems. Examples of such communication systems are: field bus, profibus, Ethernet, industrial Ethernet, FireWire, or internal PC bus systems (PCI). These bus systems are in each case designed or optimized for different fields of application and allow a decentralized control system to be built up. Process control and monitoring in automated production and especially in the case of digital drive technologies require very fast and reliable communication systems with predictable response times. A very fast and simple communication can be built up between various assemblies by means of parallel bus systems such as, for example, SMP, ISA, PCI or VME. These known bus systems are used particularly in computers and PCs.
Synchronous clocked communication systems having equidistance characteristics are known from automation technology. These are understood to be systems consisting of at least two users which are connected to one another via a data network for the purpose of mutual exchange of data or mutual transmission of data. The data are exchanged cyclically in equidistant communication cycles which are predetermined by the communication clock used by the system. Users are, for example, centralized automation devices, programming, planning or operating devices, peripheral devices such as input/output modules, drive systems, actuators, sensors, stored-program controls (SPC) or other control units, computers or machines which exchange electronic data with other machines, and particularly process data from other machines. In the text which follows, control units are understood to be closed-loop or open-loop control units of any type.
An equidistant deterministic cyclic data exchange in communication systems is based on a common clock or time base of all components involved in the communication. The clock or time base is transmitted from a designated component (master clock) to the other components. In the case of an isochronous real-time Ethernet, the clock or time base is predetermined by a synchronization master by transmitting synchronization messages.
In German patent application 100 58 524.8, not previously published, a system and a method for transmitting data via switchable data networks are disclosed, which allow a mixed operation of real-time-critical and non-real-time-critical data communication particularly data communication, based on the Internet or Intranet.